El óxido nítrico como regulador de los cambios hemodinámicos en la gestación.

SUMMARY Nitric oxide (_GCB_NO) produced by the majority of animal cells, has been considered a second messenger, since it is the result of a transduction process induced by a first stimulus. Biochemically, _GCB_NO is produced during the conversion of L-arginine to L-citruline by a reaction catalized by the enzyme nitric oxide syn-thetase. Two ixoenzymes have been characterized from this enzyme: a constitutive isoenzyme activated by hormones produced by the endothelial cells and acting on smooth muscle relaxing properties and the other, an inducible isoenzyme whose synthesis is stimulated by cytokines, and produced by macrophages. As pregnancy progresses, the concentrations of _GCB_NO, its metabolites, nitrates and nitrites, cGMP and the synthesizing enzyme, nitric oxide synthetase, increase parallelly until reaching a maximum peak before birth. It is considered that _GCB_NO is the molecule that maintains the typical vasodilated tone during prgnancy. During preeclampsia, this free radical, as well as its metabolites are found to be significantly decreased, in addition, the administration of _GCB_NO donors or of the precursor of L-arginine reverts the vascular abnormalities of this condition. The mechanism of action behind _GCB_NO on the vascular endothelium is by its stimulating effect on the enzyme cyclase guanilate, causing an increase in cGMP concentration and the relaxation of the smooth muscle. The nitric oxide generates by macrophages acts as a defense mechanism when linked with other radicals as the superoxide anion (O[sub2]). [ABSTRACT FROM AUTHOR]